The use of disposable hygienic products, such as baby diapers, adult incontinence products and feminine napkins, is widespread in developed nations. All of these products are designed to absorb body fluids efficiently and at low cost. Typically, the principal absorbent material used in the core of these products is cellulosic fluff pulp. The cellulosic fluff pulp is usually obtained by defibering a pulp sheet in a hammermill or a pin mill. Although cellulosic fluff pulp is relatively inexpensive, its high weight percentage in the final product makes it the major material cost of the hygienic disposable product. Additionally, increasing environmental concerns dictate that disposable products be lower in volume and weight so that they don't take up as much landfill space. As a result, there is an increasing need to reduce the quantity of cellulosic fluff pulp in disposable products.
One technique that has found widespread acceptance in the baby diaper industry is to incorporate one or more super absorbent polymers (hereinafter "SAP") into the cellulosic fluff pulp. The absorption capacity index (i.e., g fluid absorbed/ g of absorbent) for diaper cores containing SAP is significantly greater than that for cores containing no SAP, so the total core weight of the disposable product can be reduced while still maintaining equivalent total absorption.
Another technique for reducing the quantity of cellulosic fluff pulp in the final disposable product is to thermally bond the cellulosic fluff pulp with a small amount (e g., 2-30 wt. %) of a fusible synthetic pulp or fiber that is intermixed therewith. The resulting bonded pulp has greatly increased tensile and compressive strength over unbonded pulps. In addition, fluid absorption, particularly under load, is also increased. As a consequence of such bonding, the core weight of the disposable product can be reduced significantly, while still maintaining equivalent total absorption.
In such bonding applications, a thermal bonding device is necessary to realize reductions in core weight. Since the fluff pulp core is created during the manufacturing process, it is highly desirable for the thermal bonding device to be an integral part of the process. Although it is possible to make bonded cores separately, and then to combine them on the production line with other elements of the disposable product, such steps are economically unrealistic. Several specific bonding techniques have been proposed, but there are significant speed and safety constraints that have sharply limited commercialization of these techniques.
The presently preferred bonding technique practiced by the disposable art utilizes a "through air" system in which air, heated above the fusion temperature of the fusible synthetic pulp or fiber, is introduced into a substantially closed container. The intermixed pulp material travels around a perforated drum or on a mesh belt which is under vacuum, so that heated air is drawn through the pulp material thereby heating it above the fusion temperature of the synthetic pulp or fiber. The bonding step is critical because, at the operating speeds of present day production lines, fusion must be accomplished in a fraction of a minute, ideally two seconds or less. Fusion time can be increased by increasing the length of travel in the thermal bonder, but this approach leads to excessively large and costly bonding units.
Another alternative is to use radiant energy to bond the pulp material. However, radiant heating creates a significant safety hazard because the equipment utilized operates at temperatures above the ignition point of cellulose.
Still another technique is to use dielectric heating, but there are formidable problems with the use of this type of technology. Dielectric heating involves rapid and uniform heating throughout a nonconducting material by means of a high-frequency electromagnetic field. Commonly, this includes radio frequency (hereinafter "RF") and microwave energy. Most commercially available RF or microwave heaters are large and the product moves through them slowly. Power can be increased, but there is a limitation in this application since no arcing can be tolerated due to the extreme fire danger produced by pulp fines floating in the atmosphere. Therefore, they are not suited for an in-line thermal bonding operation useful for manufacturing hygienic disposable products.
Finally, the fusible synthetic pulp or fibers (e.g., polyolefins and polyesters) have low dielectric loss factors which make them unreactive at radio frequencies. Existing commercially-marketed synthetic fibers in general, and polyester fibers in particular, do not heat up in an oscillating electromagnetic field and cannot be thermally bonded. Indeed, polyesters are good insulators and have low dielectric or inductive loss. This property is why polyesters are useful in capacitors.
In order to overcome the above-noted problem, the costly step of incorporating a polar material in the manufacture of the fusible synthetic pulp or fiber has been considered necessary to achieve rapid dielectric heating to fusion temperatures. For example, the approach adopted in U.S. Pat. No. 4,401,708, directed to a method of bonding nonwoven fabrics using microwave energy and a polar trichloroacetic acid solvent, presents significant control problems in applying the solvent to appropriate pulp or fiber locations without excessive degradation of the pulp or fiber upon prolonged exposure to the solvent. Control problems lead to nonuniformly bonded products.
Although techniques for thermally bonding absorbent fluff pulp cores have been known for many years, their adoption by the hygienic disposable product industry has been very slow. Reasons for this situation include the unsatisfactory size, cost and safety of commercially available bonding units.
As a result, what is needed is a process to improve the speed of thermal bonding over what has previously been possible commercially. Another objective is to overcome some of the limitations that have been apparent, and even inherent, in various prior art commercial bonding techniques. Moreover, the cost-effectiveness of the process is always an important objective for any commercial operation.
Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the attached drawings and to the detailed description which hereinafter follows.